The Triangle's Cave
by Anonymistress
Summary: A Gravity Falls AU. Bill Cipher lives in a hole in the ground, and has visits from Dipper and Mabel. He doesn't seem like such a bad guy...
1. Chapter 1

There were a pair of twins, living together in a house. The elder twins, were brothers, and legal guardians of the younger ones. They ran a tourist trap called the Mystery Shack together. Stanford, or more commonly called Ford, was in charge of creating attractions for the shop, while Stanley, aka Stan, was the el facto chief, the manager. The kids were their great niece and nephew, and had somehow fallen into their lap, and were stuck there.

There were a couple of teenagers who worked there as well. Wendy, the cashier, and Soos, the janitor, repairman, and plumber.

Dipper and Mabel were the names of the twin siblings. They never really got to know their parents. The older men were kind to the children, and did not make them work. They had a higher authority, and were able to go where they pleased.

It was on a hot summer day when they found the secret of Gravity Falls.

Dipper and Mabel both knew everything in the Mystery Shack was fake. There was no such thing as magic. They'd walked the forest many times, and there was nothing whimsical about it. At least, there hadn't been. But then Dipper fell.

"AHHH!" Dipper screamed, as he took a wrong step, and started to plummet. He rolled down a hill, and stopped, plunking into some ice cold cave water. He yelled, "MABEL! ANYONE?" He stated up the way he came, from which the only light seemed to come, just barely. But then the cave seemed to glow brighter. Dipper turned slowly, and saw a little triangle, floating there, looking curious. The boy jumped a little, shocked, but not afraid. "Hello..." he said, waving at it.

"Hey, kid. My name's Bill Cipher. I haven't had a visitor in so long..." the triangle said softly, staring at him with what seemed to be a smile in his eye.

"I'm Dipper, Dipper Pines... and, well, don't get too excited. I'm not here for long..." Dipper sighed and looked down. "Do you know the way out of here?"

"Now, why'd you wanna leave? You just got here... Come on..." Bill looked a little offended, and hurt.

"I know, but... Ugh..." He touched his arm, and realized he was bleeding. He had cut himself on the way down. "Oh, man. Oh man oh man oh man..."

"Hey, hey, it's okay, Pines... Here, let me help you." Bill Cipher began to cry, and the tears that fell on the wound healed it, instantly.

"Woah..." Dipper was brought back to reality. "Wait a minute, you're real? You're magic, and you're real..."

"As real as you are, kid." Bill nodded, smiling.

"Wow. Grunkle Ford will be so impressed! They both will!" Dipper grinned. "I knew magic was real!"

Bill looked at him sadly. "I can't leave here, Pine. I can't go anywhere. I can show you where the exit is, but you have to promise me that you'll come back before long. I... I need the company."

Dipper nodded. "I promise."

The boy was so excited. He skipped all the way home. "Grunkle Stan! Grunkle Ford!" Both ended up being busy. Stan was giving a tour, and Ford was creating a wax figurine from scratch before he lost his idea. Dipper found Mabel up in the attic. "Hey, Mabel, you'll never guess what I found!"

"Fairy treasure!" Mabel answered, grinning.

Dipper shook his head. "But you're pretty close. C'mon, I'll show you!"


	2. Chapter 2

"Woah..." Mabel stared in wonder at the blue triangle.

"His name is Bill Cipher. See, I wasn't kidding when I said magic!" Dipper grinned.

"Hello, again, Pine, and I see you brought a friend!" Bill noticed the tree on Dipper's hat, and was remembering his name by that. He saw the star on Mabel's sweater, and instantly decided that was her name.* "and you must be Star!"

"No, I'm Mabel Pines, but you can call me whatever... You're so cute and small!" She was swept away by him, grinning at the very sight of him.

Dipper watched his sister with joy, glad to see her this way. "So, Bill, have you always been a triangle?" He got out a notepad. He hadn't just brought Mabel this time - he brought the notepad, and a couple of pens.

"Yeah, I think so. I mean, I can't remember when I was born." He shrugged. He didn't seem to remember anything beyond being in the cave, after a couple more questions.

"Ooh, ooh, my turn! Bill, can you do some magic?" asked Mabel excitedly.

For a moment nothing seemed to happen. But then Bill went from blue to purple, from purple to red, from red to orange, from orange to yellow, from yellow to green, and from green to blue.

The twins cheered at this feat.

"What else can you do?" asked Dipper, scribbling furiously on his notepad.

"Hmm..." Bill closed his eye, and raised his hands, and all the little rocks in the cave lifted up at once, before he slammed them back down, opening his eye.

They cheered again.

They visited him whenever they could. They'd go to the Shack after school, do their homework as quickly as possible, and run out into the forest to talk to Bill some more. The three of them began to share a deep bond. Bill was a mediator when they argued, and a confidant when they had secrets. They told him so much about the outside world - and he was ever so proud of them, but jealous, too. He wanted to leave, but he didn't know how.

"Hey, why can't you leave?" asked Dipper one day, sitting there, munching on a sandwich. "How do you know you can't?"

Bill considered this for a moment. "There's a barrier around this cave, keeping me here. I've tried to leave before. It's... It's impossible to get through, trust me." He looked down. "Can I have some of your sandwich?"

"How can you even eat? You don't have a mouth..." Dipper protested, but gave him an eighth of it.

"Hmm... Watch this." Bill closed his eye, and then it wasn't an eye. It was a mouth, and he used it to devour the sandwich in seconds.

"Wow... You're so cool, Bill..." Dipper smiled and hugged him.


	3. Chapter 3

Dipper and Mabel talked for a while about it, before agreeing - they would talk to their great uncles, and show them Bill.

It was at the dinner table. Everyone was eating. Even Waddles, snorting away.

"So, um, hey, Great Uncle Ford?" Dipper asked.

"Yes?" The man looked up.

"Mabel and I went into the woods the other day, and we found something incredible - magic actually does exist!" Dipper grinned.

"Yeah! There's this little guy named Bill Cipher who lives in a cave, and he's literally a triangle with an eye!" Mabel supplied, smiling.

Stan and Ford were silent. Until Ford growled, just a little, and said in a dangerous whisper "There is no Bill Cipher."

"But we saw him -" Dipper started.

"There is no Bill Cipher!" Ford exclaimed, slamming his fist into the table, making everything jump.

The kids look a little afraid, wondering about their decision.

"Kids, you should get to bed. And don't bring up magic ever again." Stan said, glaring at them. And so they went into the attic.

"Man, I've... They've never acted like that before." Dipper said softly.

Mabel looked down. "Was it me? Was it something I said?"

"No, no, Mabel, it's just... I don't know what it was, but it wasn't you, I promise..." Dipper held her hand.

"But what if it was?" she asked, tortured by the thought.

Ford stared at his reflection in the plate. "Bill Cipher... Why do I know that name?" he asked, and sighed.

Stan hugged Ford. "Ah, don't worry about it, I'm sure it's nothing important."

"But it is... I know it is. I know it has to be. Why? How?" Ford looked through the window in wonder.


	4. Chapter 4

Mabel found herself drawn to the cave in the night. She tried to take Bill out of the cave, pushing him, pulling him, but there was a magical barrier, trapping him here in this cavern. She frowned. "There's gotta be a way to get you out of here..." she muttered.

"I don't think so, Star. Unless..." He shook his head. "No, no, never mind."

"What?" asked Mabel, curious. "Tell me, please!"

Bill was silent. "...You should go home, Star, it's getting late, and you need your sleep. Don't forget about your tests tomorrow."

She sighed. "Okay. But you have to promise you'll tell me."

"I can't."

"Why not?"

"Because... Only Pine can do it." Bill said. "And I don't think he'd want to, anyway."

Mabel got up. "Well... Until then, I'll bring down some stuff to make this place cozy and comfortable for you!" She grinned at him, blissfully unaware of how hard that would be, starting tomorrow.

After the kids came home from school, Stan was there waiting, with steely eyes and crossed arms.

"Kids, I'm putting you to work." said Stan, looking down at them both. "You two need to learn to have responsibilities. So, Mabel, you have dishwasher duty, and Dipper, you have to do the laundry, every day."

The younger twins groaned in protest. "But-" they both started simultaneously.

"No buts! Get to work. And I don't want to hear any complaining."

They sighed, sadly, and went to their tasks. Mabel would have preferred Dipper's job, as Dipper would have liked hers, but they both had little choice in the matter. So they went on with their new chores, trying to figure out how to do them.


	5. Chapter 5

It took much longer to finish their homework, and do their chores, and Stan made sure it was dark outside before they were done. Ford and Soos began to build a fence around the area. The twins began to realize they were being put on lockdown.

In the morning, the bus would come around the Shack. But the two didn't get on. Instead, they fled into the forest, to find the cavern where Bill was. Mabel brought sleeping bags and other things, books and crafting supplies, so she and Dipper could stay down here during the night, sneak away. They were too afraid to ask again to their Grunkles about Bill or the new rules being put in place.

Dipper asked Bill. "So... You've ever heard of Stanford Pines?"

"Stanford... Pines... Sixer?" he asked, frowning. "I'm not sure why I know that..."

"Hmm... That's kinda weird. But Sixer? Are you talking about his hands?" Mabel asked.

Bill shrugged, having no good answer.

Dipper sighed. "It's okay... We'll figure this out, one way or another."

"Oh, I just remembered - Bill, that way to get you out of here you told me about that only Dipper can do!" Mabel exclaimed.

Bill whimpered a little. "Do I have to...?"

"C'mon, Bill, we won't be able to come here much anymore. The Grunkles are locking us down, not letting us go outside. We had to sneak out to get here..." Dipper said, staring at him.

Bill sighed. "Alright. I need you hold my hand. It's... It's going to be uncomfortable for both of us, but it'll be okay." He smiled with his eye.


	6. Chapter 6

They held hands. Dipper waited, and then something was different, but it was hard to tell what. "Pine... I wouldn't ask this of you, if there was any other way. But... I need a vessel. A body, to travel in, like those cars you tell me about."

"I don't understand why it can't be Mabel." Dipper frowned.

Bill sighed. "I don't know, either. But I know, for some reason, it has to be you. Here... Take my hand, and that'll seal the deal..."

Dipper trusted Bill with his life. With all his secrets. Maybe because Mabel was the only one he would ever tell, and she already knew most of those things anyway. But he was uncertain, for a moment. Something warned him, something told him not to do this.

He took Bill's hand. "I'm sorry," said the triangle, before Dipper felt himself yanked away, by Bill's hand, into the air, now weightless, floating. Bill flew into the copy of Dipper he saw, the one standing where he stood, and suddenly things returned to normal.

"Did it... Did it work?" Mabel asked.

Dipper's form, slumped on the floor, slowly stood and opened his miscolored eyes. He grinned. "It did, Star, it really did."

"Woah... What happened?" She was in shock, staring at him, a little afraid.

"Oh... I'm borrowing your brother's body. It's... It's only to escape from this cave with." He smiled.

"Oh, okay... Let's go outside, then." She suggested, uncertain about all this now. She got that feeling Dipper did.

Dipper was looking down on them, confused. "Uh, guys, I'm right here... Guys?"

But they couldn't see him, for some odd reason.


	7. Chapter 7

Bipper and Mabel walked out of the cave easily. But behind them, Dipper was now trapped. "HEY! GUYS! I CAN'T... I CAN'T GET OUT!" he yelled, terrified. But they still couldn't hear him. The brunette began to panick.

"Feel free to go ahead, Star, I'll catch up. I wanna admire nature..." Bip smiled at her.

She nodded. "Sure thing!"

Bipper grinned, finding a large rock. He looked down the whole. "Hey, Pine, you alive down there?" he called.

"Yes! Bill, please come down here, I can't... I can't leave!" yelled the boy, looking up in his ghostly form. He furrowed his incorporeal brow. "What's with the rock?"

Bill didn't tell, he showed. By dropping it on the hole, covering it completely. He chuckled, and caught up to Mabel.

"Welp, Bill's free, let's go home." Bipper said with a soft, innocence smile. They slipped under the fence, heading back to the Shack, feeling successful for different reasons.

"BIIIIILL! MAAAAABEL! HEEEEEEEEEELP!" Dipper screamed, but was completely unheard. He yelled for the rest of the night.


	8. Chapter 8

Dipper kept yelling, in darkness. There was nothing to do, but yell, scream. He couldn't get out, he couldn't see. It was dark, and cold, but his throat never seemed to hurt, and he wasn't hungry, or thirsty. He was perfect... Except he was alone. "MABEL!" he called again, tears seeping into his voice.

Again, silence, except for his own echoing voice.

He began to cry, and his form slowly began to change.

Time passed quickly. Not that he had anything to tell time with. There was no light down here, except for his own. The light he cast whenever he wished to, and could dim as he wished. He could shine a variety of colors, effortlessly.

He wished it wasn't so. He would have given this newfound power for something to do, to practice, anything. There was nothing in this cave, nothing at all, and after yelling for what seemed like forever, he began to realize he wasn't going to be heard.

Bill had tricked them, it dawned on him. Bill had betrayed him and Mabel, just so he could get out of the cave.

But Dipper understood now. The cave was horrible. Small, confined... He felt power brewing inside of him, destructive power.

The boy sometimes would yell, or sing. He talked to himself a lot. That was a way to pass the time.

He would tell stories, stories of handsome princes and beautiful princesses, of love and betrayal. He talked about Wendy, and Mabel, and Stan, and Bill. He talked about everyone he could think of, found a way to incorporate them into the story. And of course, he was the hero. The knight in shining armor to save Wendy, the damsel in distress.

They were a distraction, the stories. A way to forget that he was trapped twenty feet beneath the Earth without anyone having the slightest clue he was down here.

He would often wonder how much time had passed. Seconds, minutes, hours, days, weeks, months, years, decades? There was no way to tell, and it boggled him.

He found he could float, shortly, and in a puddle he saw his reflection. He wasn't human anymore, he realized. No, he had two arms, two legs. He had a wonderful voice, one that didn't crack anymore, but... He was an upside down blue triangle, and he had a single eye.


	9. Chapter 9

Mabel knew something was wrong about her brother ever since they liberated Bill. He didn't act the same at all. No, he had turned into a masochistic, sadistic lunatic. The kid went around stabbing forks into his arms for fun.

Her Grunkles dismissed his behavior as that of a teenager's. They punished the twins severely for sneaking out, and made them promise to never visit the cave again. They did, wholeheartedly. Well, at least, Dipper did.

Mabel went back to the opening of the cave and found a large rock in place, blocking it off. She tried, but was simply not strong enough to lift it. The boulder was immobile, neither her nor Wendy could lift it.

The girl decided to relax. Bill was gone, she told herself. Everything was fine! And they didn't have to sneak out anymore. That was good, too, wasn't it? Yes, everything was quite fine.

Until she woke up. "Good morning, world!" she cheerfully chirped, and glanced over to Dipper's bed. "Good morning, Dipper!"

His form laid in the bed, unmoving. Which was better than what she could say for some nights. She had found him drunk before, self harming and asking why it didn't hurt. She ignored the memory, as she jumped out of bed and ran onto Dipper's, and began jumping. "WAKE UP, WAKE UP, WAKE UP!" she cheered, grinning, until the sheets parted and Dipper was not there, only pillows to mimick his form.

"Dipper...?" she asked, feeling foolish. Why wasn't he here? Where did he go? Quickly, she searched every inch of the attic, and then the main floor. Nothing. Which was extra odd. He had never done this before, never disappeared without a trace, and never left something to pretend he was there.

Mabel went to her Grunkles' bedroom. The brothers shared a room, but slept in separate beds, much like Dipper and Mabel.

"Guys! Dipper's missing!" she yelled.

Stan woke up almost immediately. "What? What do you mean, missing?"

"I mean, I woke up, and he wasn't in his bed!" she said, fear in her eyes.

Ford grunted, waking up. "It's too early for this..." he muttered, donning his glasses.

Stan ignored his brother. "Are you sure he isn't here, Mabel?"

She nodded, and ran off to check the house once more.

When she returned, the twins were fully dressed and ready to go. They went about the house, calling his name, for a while, until Stan said, "Did you check the roof?"

She frowned, and shook her head, and rushed off to do so. And when she did, she was met with a disturbing sight - the sky was red, and a giant X crossed it, blotting out where the sun should be. Her quavering, panicked voice screamed, "DIPPER! WHERE ARE YOU?" but there was no answer, but for the echoes and unsettling silence.


End file.
